If you look at the propeller arc on some model planes, you'll notice that it's invisible because the prop is turning so fast. The same goes with a full-scale Cessna when you are in the cab behind the rotating prop. Since this device has no center axis, that means there's no center point of rotation that the eye can see. Thus, visibility of the craft is significantly reduced. It's an interesting machine to say the least.

I saw this at AUVSI and spoke for quite some time with the designer. It is currently in prototype phase and lacks an autopilot or even any sort of orientation system for r/c control (i.e. synchronized LED) but the free flight tests seem very promising.

At first glance it may seem impossible to adapt an autopilot, control system or video camera to such a craft but these adaptations are actually quite simple. Any standard helicopter-type autopilot will work with just minor changes to accomodate the rotation and cyclic control. The control system is simple cyclic power applied to the motors using a custom-made rapid-response brushless controller. The video system is most fascinating: rather than capture a 640x480 image for example, it captures a 1x480 vertical strip and stitches it together to create a 360 degree panoramic at ~1000 rpm. The ground operator simply chooses which direction to look and the craft sends that section of video in a standard rectangular format.

Images

Interesting ship, but seen things like that before... Flashy, but doesn't quite work...
I would really like to see the onboard video.
Looks like a good candidate as a investor magnet in the pink sheet's.
Kinda like the flying car or other really cool things that do not work.....

At first glance it may seem impossible to adapt an autopilot, control system or video camera to such a craft but these adaptations are actually quite simple.

And a second glance confirms that notion. I'm skeptical that a gyro/acceleromater based autopilot can be used. Many commercial autopilots use gyros with a range of +-300 deg/sec, and the rotation of the phantom is clearly a lot faster than that -- so the yaw gryo is out. Furthermore, since the center of rotation of the phantom is not inside the fuselage, using accelerometers on the x&y axes will be tricky. There will be an offset measured by the gryos due to centripetal acceleration, and this will be hard to actually determine since the yaw gyro is not useful. Also, the accelerometers must have a large enough range to accomodate the offset and still provide useful data (depending on distance from center of mass). No, I think it is quite a challenge to integrate an IMU based autopilot into this vehicle, and that is why they are using a horizon detection based leveling system. It might not be impossible, but certainly it is not just "drop in a heli autopilot and go" type situation.

However, I think the horizon sensor and gps based navigation would be adequate to steer it around the sky (assuming the horizon leveler performs well).

Hmmm... true, that's about 6000 deg/s, well out of the range of common miniature gyros. Horizon detection will work fine for levelling but for navigation the rotation rate must be known with some accuracy. A standard accelerometer mounted at a known distance from the c.g. should give an accurate rpm measure but you're right, this is some pretty substantial modification to common autopilot code.

The designer told me that he currently has full r/c control but is not yet measuring the rotation rate, so any cyclic motor control would have to be done with a very skilled thumb. He made it sound like he was just a few hours of work away from installing a gyro and LED in the thing and starting r/c navigation. Made sense to me, as it is quite simple to do that, though I neglected to calculate the rotational speed involved.

Actually, now that I think about it, it should be possible to make an extremely effective autopilot using nothing more than a common $10 MEMS accelerometer. Measuring the centripetal acceleration will give the rpm and then measuring the gravity vector relative to the rpm will give absolute pitch and roll angles as well as climb rate, pitch/roll rates, and even rotational accleration all with far more precision, speed, and robustness than any conventional IMU. LEDs would make r/c navigation simple and the synchronized video would do the same for RPV. The inevitable yaw drift resulting from a slightly imprecise rpm measure would only affect navigation and may likely be unnoticable to the opearator - the only big challenge would be autonomous navigation as I don't think any common magnetometers would work at that speed and navigation would be pretty sloppy without one.

There is a problem with measuring both the gravity vector and RPM with accelerometers. There are four unknowns in the problem, [x,y,z] components of gravity and then RPM, while there are only three sensors. (assuming a steady state)

Unfortunately, it is also not possible to make an IMU using only accelerometers, even on just a simple aircraft. If you just use accelerometers, as soon as you deviate from a steady, non-accelerating state, you will no longer be able to measure the gravity vector because there is no way differentiate gravitational acceleration from acceleration due to other forces (aerodynamic, etc..). Accelerometers can only ever measure local reaction forces. Imagine if the vehicle is in a free fall (before terminal velocity is reached) -- the accelerometer would measure nearly zero. If the vehicle then accelerates downward faster than gravitational acceleration,the accelerometer based IMU will report the gravity vector reversed from reality.

I've been thinking about using horizon detection only. Would it be possible to come up with some sort of feedback law based only on GPS position? What I'm thinking is you have the nominal rotation speed (you could even base this on throttle setting of the two motors) with which you come up with a basic cyclic control law. Then the phasing of the cyclic control is adjusted according to the difference between commanded and actual heading information you receive from GPS. Since GPS heading is based on physical motion (and not a megnetometer), the rotation should not cause a problem. The part I'm not familiar with is the horizon detection....could it be used to command a certain angle with respect to the horizon?

Edit: there may even be another option: a 3 axis magnetometer. Just a quick search shows that Honeywell makes a small 40gram unit that has a maximum sample rate of 154 Hz, which should be adequate assuming the phantom operates below 2000 RPM. However, there may be a vibration issuse and the two motors may cause magnetic interference (but maybe this can be calibrated out(?)). I'm not really familiar with magnetometers, so I don't know how fast they react to changes in orientation, but there are projects that use only a 3-axis magnetometer for attitude sensing with good results.

Actually, there are not 4 unknowns as the rotation ties everything together under the assumption that the rotation rate is so much greater than the pitch/roll rates that you can assume a constant attitude and rpm throughout each revolution. In fact, only two accel sensors would be needed since the rotational acceleration can be derived from the rpm change and the complete gravity vector can be built from the rotating 2D vector. Of course a 3-axis accel would be preferable, but theoretically even just a single radial accel could give rpm and attitude with perfect accuracy and decent robustness (only limited to upright flight). The radial acceleration gives rpm and any cyclic readings give the exact attitude.